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Background    Haemophilus parasuis   is a common porcine respiratory pathogen that causes high rates of morbidity and mortality in farmed swine. We performed a molecular characterization of antimicrobial resistance genes harbored by  H. parasuis  from pig farms in China.    Methods  We screened 143  H. parasuis  isolates for antimicrobial susceptibility against six fluoroquinolone antibiotics testing by the broth microdilution method, and the presence of 64 antimicrobial resistance genes by PCR amplification and DNA sequence analysis. We determined quinolone resistance determining region mutations of DNA gyrase ( gyrA  and  gyrB ) and topoisomerase IV ( parC  and  parE ). The genetic relatedness among the strains was analyzed by pulsed-field gel electrophoresis.    Results  Susceptibility test showed that all isolates were low resistance to lomefloxacin (28.67%), levofloxacin (20.28%), norfloxacin (22.38%), ciprofloxacin (23.78%), however, high resistance levels were found to nalidixic acid (82.52%) and enrofloxacin (55.94%). In addition, we found 14 antimicrobial resistance genes were present in these isolates, including  bla  TEM-1  , bla  ROB-1 ,  ermB, ermA, flor, catl, tetB, tetC, rmtB, rmtD, aadA1, aac(3′)-llc, sul1, and sul2  genes. Interestingly, one isolate carried five antibiotic resistance genes ( tetB, tetC, flor, rmtB, sul1 ). The genes  tetB ,  rmtB,  and  flor  were the most prevalent resistance genes in   H. parasuis  in China. Alterations in the  gyrA  gene (S83F/Y, D87Y/N/H/G) were detected in 81% of the strains and  parC  mutations were often accompanied by a  gyrA  mutation. Pulsed-field gel electrophoresis typing revealed 51 unique patterns in the isolates carrying high-level antibiotic resistance genes, indicating considerable genetic diversity and suggesting that the genes were spread horizontally.    Discussion  The current study demonstrated that the high antibiotic resistance of  H. parasuis  in piglets is a combination of transferable antibiotic resistance genes and multiple target gene mutations. These data provide novel insights for the better understanding of the prevalence and epidemiology of antimicrobial resistance in  H. parasuis .

Characterization of antimicrobial resistance genes inHaemophilus parasuisisolated from pigs in China